1. Field of the Invention
The present invention relates to a fault diagnosis device applied in a passenger protection apparatus of an airbag apparatus for vehicle use, a seatbelt pretensioner, or the like, and more particularly, to a fault diagnosis device applied in a passenger protection apparatus provided with a plurality of activating elements to activate the passenger protection apparatus.
2. Related Art
Conventionally, a passenger protection apparatus for vehicle use detects acceleration of the vehicle with a sensor and detects collision of the vehicle on the basis of the detected acceleration. At the time of vehicle collision detection, the passenger protection apparatus is caused to be activated to protect a passenger. Because the passenger protection apparatus must operate reliably at the time of vehicle collision, it is normally provided with a self-diagnosis device so as to be able to inspect for the presence or absence of an abnormality in various areas of the apparatus. With regard to the activating element, which is the most critical structural portion, the presence or absence of an abnormality is determined by measuring whether a resistance value of the activating element is normal, as in, for example, Japanese Patent Application Laid-open No. 4-28779. In addition, inspection of other aspect is executed and the presence or absence of respective abnormalities is determined.
Normally, a minute monitor current passes through to the activating element. A resistance value is calculated from the voltage values generated at the activating element during conductance of the monitor current, and the presence or absence of abnormality is determined. In a case where this passenger protection apparatus exists for example at a driver's seat and a passenger seat, a plurality of activating elements are disposed. In this case, the respective activating elements are inspected to confirm that the respective activating elements operate normally.
Because of the constitution of the activating elements, a large monitor current cannot be caused to flow. Moreover, because a resistance value of the activating element is normally low (approximately 2.2 Ohms), the voltage value produced by the monitor current is extremely small. For these reasons, an operational amplifier (op-amp) is utilized to amplify the voltage value. However, the voltage value amplified by the op-amp includes an error due to an offset voltage in the op-amp. To remove the offset voltage included in the voltage value, two types of monitor currents are delivered to the activating element and causes two types of voltage values to be generated. Because the two types of voltage values include the same error, the error can be removed by subtracting one voltage value from the other voltage, as is disclosed in Japanese Patent Application Laid-open No. 6-24289. Accordingly, the resistance value of the activating element is determined from the voltage value from which the offset voltage has been removed.
However, in a passenger protection apparatus having a plurality of activating elements, it is impossible for the activating elements to operate independently, and so in many cases a plurality of activating elements are connected in parallel with respect to a control circuit. In a case where activating elements are connected in parallel, the monitor current passes simultaneously through the activating elements at a time of fault diagnosis. For this reason, the monitor current is divided and conducted to the respective activating elements. As a result, the voltage value due to the monitor current becomes increasingly smaller. Additionally, if the activating elements are connected in parallel, there is a chance that the monitor current will become uncertain with respect to each activating element. In addition, there is also a chance that an abnormality in one activating element will affect the monitor voltage value of other activating elements. Consequently, it is not preferable simply to connect activating elements in parallel.